“One of the simplest models of directional selection, truncation selection, where the bottom (or top) x% for a trait fail to reproduce is easy to model and produces something that closely fits observed situations.

“Say those 1 standard deviation below average for a trait fail to reproduce – roughly the bottom 16%. (In terms of numbers, this isn’t far off from the fraction of people that fail to reproduce in modern America.)

“If we assume those 1 s.d. below average fail to reproduce, then the mean of the parental population (assuming trait in question is normally distributed) is the mean of truncated bell curve cut at -1 s.d. which you can find (with some…fancy math) to be +0.29 sd.

“Since the additive heritability of most traits is 0.5, the response to selection in that case is 0.29 * 0.5 = 0.145 sd/generation. If this were IQ, that would correspond to a ~2.2 point gain per generation. Assuming sustained selection, the population mean would move one whole standard deviation in just 7 generations (or about 200 years)! I mentioned IQ, but this will work just as well for any quantitative trait with a similar additive heritability, including the personality traits associated with a fine manorial serf – which you [could] model collectively as a ‘manorial quotient’ (MQ).“

“Outbreeding has produced an evolutionary shift to the right (maybe to the upper right) for NW Euros on this map. If we assume they started about where the Slavs are now, that means they moved +2 or +3 s.d. over the course of the relevant evolutionary time. Such a change (given the case of strong, sustained directional selection) could take as little as 400-600 years, given the formula above.“

and that, dear readers, is how you make hbd chick smile. (^_^)

edit: oh! jayman promises that there’s gonna be more like this in an upcoming mega-post on his blog, so stay tuned!